"In a derby you have to be careful - the second yellow card is a stupid yellow card," Van Gaal told BBC Sport.

"The sending-off is not one of those things. As a player you have to control your aggression.

"I didn't see the first yellow but with the second, you know you already have a yellow, so have to handle it differently. I said that to the players."

Despite Smalling's dismissal, United improved after Aguero's goal, with Robin van Persie and Angel di Maria both drawing good saves out of City goalkeeper Joe Hart.

MOTD2 analysis

Danny Murphy: "Especially with them playing against 10 men, you might not expect that City would drop deeper and deeper when they went 1-0 up rather than continue pushing forward in search of a second goal, but there were reasons they had that mentality."

"The red card had an influence on the game but, despite being 10 men against 11, we played better in the second half than the first because of willpower of this team," added Van Gaal.

"We are very close. You saw it against Chelsea and now against Manchester City. They are the two best clubs in this league and the difference is zero I think.

"When you see what we did with 10 men you can be proud as a coach."

Van Gaal also defended midfielder Marouane Fellaini against accusations that he spat at Aguero after the striker went down under a first-half challenge.

Former Wales international Robbie Savage:

"Manchester United are level on points with West Brom. Don't get me wrong, I think Alan Irvine is doing a great job there, but United have spent £150m on new players and that is unacceptable. When are we going to start asking serious questions of Louis van Gaal in press conferences because I haven't heard it yet."

"The TV has shown that he is shouting and sometimes when you shout there's a little bit of saliva with it," Van Gaal said. "I don't think that he's a spitting figure."

The Football Association will wait to see if referee Michael Oliver mentions the incident in his report before deciding whether to take action against Fellaini.

If the alleged spit was not seen by the referee or any of his assistants, the FA video review panel could take disciplinary action against the Belgian if they think it was a deliberate act.